(1) Field of the Invention
The present invention pertains to a load directing trunnion mount for a linear actuator that is constructed to receive all of the tensile forces exerted on the actuator shaft or lead screw of the linear actuator. In this manner, the load directing trunnion mount relieves a transmission drive element (for example, a drive gear or a sprocket or pulley) and the transmission housing enclosing the drive element from tensile forces exerted on the actuator shaft during use of the linear actuator.
(2) Description of the Related Art
A linear actuator of the type with which the present invention is concerned is basically a mechanism that converts rotational movement into linear movement. The mechanism includes a drive assembly that can be controlled to selectively rotate a screw threaded shaft or lead screw of the actuator in opposite directions. The drive assembly typically includes a motor, for example an electric motor, and a transmission coupling the motor output shaft to the lead screw. The transmission assembly can be a gearing assembly, a sprocket and chain assembly or a belt and pulley assembly. The linear actuator also includes a nut assembly that is mounted on the lead screw of the actuator for linear movement of the nut assembly along the lead screw in response to rotation of the lead screw. By rotating the lead screw in opposite directions of rotation, the nut assembly moves in opposite linear directions along the length of the lead screw.
FIG. 1 shows one operative environment of a linear actuator of the type described above. It is emphasized that the particular use made of the linear actuator in FIG. 1 is illustrative only. Linear actuators of the type shown are used in a variety of different environments where it is desired to convert reciprocating rotary movement to reciprocating linear movement.
FIG. 1 is a schematic representation of a cross section through a supporting frame of an adjustably elevating exercise treadmill of the prior art. The figure shows a cross section through a frame member 12 of the treadmill frame that supports the running deck (not shown) of the treadmill. The right-hand end of the frame member 12 is shown broken away, but the right-hand end of the frame member would rest on the supporting surface 14 on which the exercise treadmill is placed. The left-hand end or the elevating end of the treadmill frame is supported on a pair of bell cranks 16, only one of which is shown in FIG. 1. The bell cranks 16 are mounted to opposite frame members 12 of the frame by a pivot shaft 18. One arm 22 of each bell crank extends downwardly from the pivot shaft 18 to a cylindrical roller 24 mounted on the distal end of the arm. The roller 24 supports the forward or left-hand elevating end of the treadmill frame on the support surface 14. The second arm 26 of each bell cranks extends upwardly from the pivot shaft 18. The distal end of each second arm 26 is mounted by a pivot connection 28 to a nut assembly 32 mounted on the lead screw 34 of the linear actuator. The nut assembly 32 can have internal screw threading that is complementary to the external screw threading of the lead screw, or can be a recirculating ball type nut assembly or other type of nut assembly commonly employed with linear actuators of this type. The lead screw 34 is mounted for rotation inside a transmission housing 36 by a pair of bearings mounted in opposite walls of the transmission housing. The transmission housing 36 contains a transmission mechanism that includes a drive element, for example a gear, sprocket or pulley, that is secured to the lead screw 34 for rotation therewith. The drive element is driven by the transmission contained in the transmission housing 36 which in turn is driven by an electric motor 38. Exercise treadmills of this type commonly have controls (not shown) that can control the electric motor 38 to drive the transmission and ultimately the lead screw 34 in opposite directions of rotation. By controlling the rotation of the lead screw 34 in two directions, linear movement of the nut assembly 32 across the lead screw 34 is also controlled. The linear movement of the nut assembly 32 across the lead screw 34 controls pivoting movement of the bell cranks 16 about their pivot shaft 18 which in turn controls elevating movement, of the left-hand end of the treadmill shown in FIG. 1. For example, operation of the electric motor 38 to rotate the lead screw 34 causing the nut assembly 32 to move to the left as viewed in FIG. 1 will result in the bell cranks 16 rotating in a counterclockwise direction about its pivot shaft 18 and thus elevating the left-hand or forward end of the treadmill frame shown in FIG. 1. Controlling the electric motor 38 to rotate the lead screw 34 in the opposite direction causing the nut assembly 32 to move to the right as shown in FIG. 1 will cause the bell cranks 16 to move in a clockwise direction about their pivot shaft 18 resulting in the lowering of the treadmill frame shown in FIG. 1.
Transmission housings 36 of the type shown in FIG. 1 are commonly connected to the frame members 12 of the treadmill by a pivot pin 42 extending through a hole in a flange or flanges 44 of the transmission housing and a hole in a flange 46 mounted on the treadmill frame. When a load is placed on the treadmill, for example, by a jogger on the treadmill, the load is transmitted through the bell crank 16 to the lead screw 34 as a tensile force on the lead screw. This tensile force exerted on the lead screw 34 is transmitted to the transmission housing 36 and ultimately to the flanges 44 of the transmission housing that are connected by the pivot pin 42 to the flange 46 of the frame.
FIG. 2 shows a detailed view of a prior art linear actuator of the type employed in a treadmill such as that shown in FIG. 1. In FIG. 2, like parts of the linear actuator described in reference to FIG. 1 have the same reference numbers. FIG. 2 shows a distal portion 52 of the lead screw that extends outside the transmission housing 36 and has the nut assembly 32 mounted thereon. An opposite proximal portion 54 of the lead screw extends into the transmission housing 36. FIG. 3 is a partial view showing the prior art transmission housing 36 in cross section and the proximal portion 54 of the lead screw mounted in the transmission housing as well as the drive element mounted on the lead screw proximal portion.
Referring to FIG. 3, the transmission housing has a first end wall 56 with a first shaft opening 58 passing therethrough. An opposite second end wall 62, shown to the left in FIG. 3, encloses an interior volume 64 of the transmission housing with the first end wall 58. A cylindrical recess 66 is formed into the second end wall 62. The recess 66 is concentric with the first opening 58 through the housing first end wall 56. The proximal portion 54 of the lead screw extends through the first opening 58 of the housing and into the cylindrical recess 66 of the housing second end wall 62. Beginning at the right hand end of the lead screw proximal portion 54 shown in FIG. 3, the proximal portion is mounted for rotation in the first opening 58 by a bearing or bushing 68 mounted in the opening. A circular washer 70 is then mounted on the proximal portion 54 of the lead screw. The washer 70 seats against the bushing 68 and an annular shoulder 72 formed in the interior of the transmission housing first end wall 56. A thrust bearing 74 is then mounted on the lead screw proximal portion 54 seating up against the washer 70. The drive element is then mounted on the lead screw proximal portion 54. In FIG. 3, the drive element is a gear 76, but the drive element could be a sprocket for a chain drive or a pulley for a belt and pulley drive, depending on the particular transmission employed. The gear 76 has a circular recess 78 formed into the left-hand side of the gear as shown in FIG. 3, and then a slot 82 that is further recessed into the gear from the circular recess 78. The slot 82 extends across the center of the gear. A pin 84 is inserted through a pin hole 86 that passes through the lead screw proximal portion 54. The pin 84 is received in the slot 82 and thereby secures the gear 76 to the lead screw proximal portion 54 for rotation therewith. A first circular spacer 92 is then mounted on the shaft and is partially positioned in the circular recess 78 of the gear. A second spacer 94 or thrust washer is mounted on the shaft between the first spacer 92 and an interior surface of the transmission housing second end wall 62. The proximal portion 54 of the lead screw then extends into a bearing or bushing 96 that mounts the proximal portion 54 of the lead screw adjacent its proximal end 98 in the cylindrical recess 66 of the transmission housing second end wall 62.
The two bearings 68, 96 mount the proximal portion 54 of the lead screw for rotation in the respective first 56 and second 62 end walls of the transmission housing. The thrust bearing or thrust washer 74 transmits any tensile forces exerted on the lead screw 34 from the gear 76 to the thrust washer 70 and ultimately to the annular shoulder 72 of the transmission housing first end wall 56. In FIG. 3, the path of tensile forces exerted on the lead screw 34 is represented by the darkened line 102. As shown in FIG. 3, the tensile forces are first transmitted from the lead screw 34 to the pin 84 that secures the gear 76 to the proximal portion 54 of the lead screw. The pin 84 transmits the tensile forces to the gear 76 which then transmits the tensile forces through the thrust bearing 74, the thrust washer 70 to the annular shoulder 72 in the first end wall 56 of the transmission housing. The tensile forces transmitted to the transmission housing are then transmitted through the first end wall 56 of the housing to the second end wall 62 and ultimately to the pair of flanges 44 that are mounted by the pivot pin 42 to the frame flange 46.
Prior art linear actuator mountings of the type shown in FIGS. 1-3 have experienced several different types of failures when subjected to the repeated poundings of a relatively heavy jogger running on the treadmill supported by the linear actuator. The repeated pounding of the jogger on the treadmill produces repeated tensile forces exerted on the lead screw 34 that are transmitted through the transmission housing 36 to the housing flanges 44 in the manner described above. This has resulted in the transmission flanges 44 pulling and bending the second end wall 62 of the transmission housing away from the first end wall 56 of the transmission housing or to the left as viewed in FIG. 3. This can result in a bowing of the transmission housing second end wall 62 where it joins with the transmission flanges 44. In severe cases, the bowing of the second end wall 62 can result in the lead screw proximal portion 54 disengaging from inside the bearing 96 mounted in the second end wall 6 or in a cracking or splitting of the second end wall 62 in the area of the transmission flanges 44. Reinforcing the connection of the flanges 44 to the second end wall 62 of the housing, for example by adding gussets between the flanges and the end wall or by thickening the material of the end wall, would overcome this problem but would also increase the cost of manufacturing the trunnion mount for the linear actuator.
Prior art actuator mounts of the type disclosed in FIGS. 1-3 have often been constructed with plastic drive gears 76 to reduce their cost. However, the plastic drive gears 76 have also been known to fail as a result of repeated tensile forces exerted on the lead screw. A series of repeated tensile forces exerted on the lead screw 34 and transmitted through the pin 84 to the plastic gear 76 has resulted in cracking and splitting of the gear. This problem could be overcome by replacing the plastic gear 76 with a gear constructed of metal, however this would also increase the cost of manufacturing the trunnion mount.
In prior art trunnion mounts of the type disclosed in FIG. 3 that employ a metal drive gear 76 to avoid the problems of the splitting of plastic drive gears, the repeated tensile loading of the lead screw 34 is still transmitted to the pin 84. This often results in the pin 84 cracking in the slot 82 of the gear 76 resulting in a failure of the driving connection between the gear 76 and the lead screw 34.
What is needed to overcome the problems of the actuator mount described above is a way of transmitting the repeated tensile loading of the lead screw 34 to the frame flange 46 of the treadmill frame while avoiding transmission of the tensile forces to the drive element of the actuator transmission or to the transmission housing enclosing the drive element.